The intrinsic stiffness of compliant actuators, in particular tunable stiffness, is dynamically adjusted in tunable mechanical devices. The benefit of this approach is the ability to adjust the passive mechanical properties of an actuator using very simple and energy conservative controllers.
In the past two decades increased interest has been devoted to developing ‘compliant’ robotic systems. Compliance in robotics implies ‘give’ or ‘softness’ in what is typically a rigid, linked system. In early industrial robot applications, compliant systems have allowed robots to perform force sensitive tasks (e.g., surface grinding) while remaining stable during their operation. More recently, interest in developing ‘wearable’ robots or exoskeleton systems has been demonstrated. The typical role of a ‘wearable’ robotic system is to enhance a person's strength. Compliance in this type of application is imperative to ensure safety for the operator, as the robotic system is not tucked safely away behind a cage, as in a factory floor robot.
The process of interfacing robotics directly onto humans introduces design issues of weight, power consumption and again safety. To meet the challenges laid by these constraints, actuators based upon spring concepts offer a promising solution. Unlike traditional motor approaches, spring based actuators are inherently compliant, energy conservative and lightweight. Through manipulation of an actuator's ‘effective’ structure, variations in actuator stiffness can be obtained. A ‘Force Suit’ constructed from these actuators can be created, thus enabling the disabled or weak to regain lost functionality and independence in their everyday lives.
In typical-direct drive examples of background art in this area, a rotary or linear electric motor is often used to change the intrinsic stiffness of a joint by modulating the torque of the electric motor based on a feedback signal measuring the position of the joint. However, limitations of the background art in this area include the need for using electrical energy to constantly modulate the joint stiffness, the difficulty of storing this electrical energy, the low power-to-weight ratios for standard electric motors, and the very high intrinsic stiffness of the motor that ultimately must be lowered for safety reasons when applied in wearable applications. In the field, researchers have developed wearable robots based on heavy direct-drive motors or heavy direct-drive hydraulic systems.
Researchers in this area have used lightweight, powerful artificial muscles based on pneumatics. The disadvantage of these systems is the need for a pressurized air source. Still in another example, researchers have designed series elastic actuators based on a motor, transmission, spring, force sensor, and feedback. The feedback signal is used to modulate the torque of a motor using a complicated control law. Again the disadvantage is the need for a heavy transmission and the reliance on modulating the torque of an inherently stiff motor.
Specific examples of background art in this area are disclosed by U.S. Pat. No. 6,681,908 (Davis) and U.S. Pat. No. 6,676,118 (Chou). In particular, Davis discloses a tuned mass damper (TMD) that is adjustable by utilizing an adjustment screw that is retracted or advanced, changing the number of active coils in a spring that engages a damping mass in a sealed TMD. The adjustment screw changes the spring rate and the natural frequency of the spring-mass combination.
However, the versatility of Davis for actuator applications is limited because the means for adjusting the number of active coils (i.e., the adjustment screw) does not compress, advance, retract or move with a screw motion or translate the spring. By not allowing the spring to compress, advance, retract, or in general move with a screw motion, the spring cannot be used as an actuator where the position of the actuating link can be adjusted. In addition, a further limitation of Davis is that only the adjustment screw moves whereas the spring itself is fixed.
Chou discloses an adjustable casing for a helical spring, such that the helical spring mounted in the adjustable casing can be freely adjusted to a desired modulus of elasticity. In particular, Chou discloses that the characteristics of the helical spring can be changed through changing the number of active coils subjected to a compressing force or a stretching force. However, the versatility of the helical spring disclosed by Chou is limited because the spring is fixedly attached to the casing and the casing can only be adjusted manually. In contrast to Chou, a versatile spring actuator allows the number of coils in the spring to be manually or automatically changed.
Therefore, there is a need in the art for an actuator that requires less power input, less weight and consumes less energy than required by direct drive examples of the background art. Further, there is a need in the art for a class of compliant actuation concepts, referred to as “structure controlled stiffness,” that are based upon more versatile means of manipulating the internal structure of an actuator to effect a physical change in device stiffness. Moreover, there is a need in the art for an actuator that can be applied to the development of “wearable” systems that will eventually provide strength augmentation to humans.